This invention relates to the drilling of generally horizontal boreholes in subterranean formations, and more particularly to an improved method to controlling such a borehole as it is drilled to maintain it within the upper and lower boundaries of the formation being drilled. The invention is particularly applicable to drilling of degasification boreholes in coal seams in advance of mining to reduce the methane concentration in the working area during mining of the coal seam.
The use of rotary drilling to form long horizontal gas relief holes in coal beds is known in the art as a means for degasifying a coal bed in advance of mining. These gas relief holes are either vented or connected to a vacuum source to remove methane from the coal bed. The greatest problem encountered in drilling these gas relief holes is that of maintaining the bit trajectory parallel to the coal bed such that the resulting holes are actually through the coalbed rather than through an underlying or overlying formation.
It is often desirable in degasifying a coal bed to drill a series of gas relief holes for distances of 300 meters or more into the coal bed. Maintaining the bit on a horizontal trajectory parallel to the bedding planes of the coal bed is difficult. The natural tendency of the bit during horizontal drilling is to arch downward due to the forces of gravity. Other factors such as inclusions in the coal bed may cause the bit to tend to deviate from the plane of the coal bed.
Prior to this invention, there were several approaches taken to maintain the bit trajectory along the desired path. The primary factors affecting the direction of drilling are bit thrust and bit rotational speed. As a general rule, decreased thrust and increased rotational speed tend to cause a downward bit trajectory, while increased thrust and reduced rotational speed tend to cause an upward trajectory. Other factors such as formation hardness, bit type and drill rod weight can affect the trajectory of a borehole. An experienced operator must consider the above as well as other factors in controlling the trajectory of a borehole.
There are known methods for determining the position of a borehole relative to the boundaries of a coal bed. These methods are quite effective, but they are time consuming and expensive, and any drilling process which can reduce the number of borehole surveys necessary is much to be desired.
The state of the art to which the present invention pertains is set forth in detail in a Bureau of Mines Report of Investigations published in 1975, numbered 8097 and entitled "Rotary Drilling Holes in Coalbeds for Degasification," by Cervick et al, available in the U.S. Department of Interior library. That report describes the use of rotary drill bits attached to drill rods and maintained in a desired trajectory by a combination of bit thrust, rotational speed and drill rod stabilizer spacing. That report further notes that locating a stabilizer near the drill bit will cause a slight upward trajectory to the bit with proper drill thrust and bit rotational speed, and further notes that a downward trajectory can be obtained by locating a stabilizer several meters behind the bit. However, relocating a stabilizer to facilitate a change in bit trajectory due to change in bed dip or to the bit straying out of the coal bed for any reason involves removal of the entire drill string. Such a procedure is time consuming and unproductive.
More recently, attempts have been made to avoid the need for frequent removal of the drill string to relocate stabilizers. A device for accomplishing this is described in U.S. Pat. No. 4,108,256.
While the prior art devices and techniques have been successful to some extent in controlling the trajectory of generally horizontal boreholes in subterranean formations, there has been a continuing need for an improved method of accomplishing this objective in a manner which can be readily taught to unskilled operators, utilizing uncomplicated equipment.